Abstract: An apparatus for thermally processing a wafer at an elevated temperature, in which a processing position is bounded at least on one side by a housing part which, in use, is at an elevated temperature, the apparatus being provided with measuring means for determining the position of a wafer in the processing position before and/or during processing, the measuring means being provided with at least one signal processor and at least one signal conductor, the or each signal processor being located at a distance from the processing position in an area of lower temperature, the signal conductor extending through the housing part being at an elevated temperature and extending from a measuring point in or near the processing position to the signal processor for transmitting to the signal processor contactlessly obtained measuring signals, the at least one signal conductor, at least the part extending through the housing part being at an elevated temperature, being heat resistant.